Heavy metal sulphide deposits and geochemical surveys for heavy metals in New Zealand

The ultimate primary source of heavy metals in the environment is crustal rock and the mineral deposits it contains. Background levels in the earth's crust of the main "toxic" heavy metals lie in the ranges of: lead, 1-20 ppm; cadmium, 0.01-0.03 ppm; mercury, 0.01-0.09 ppm; and arsenic, 1-13 ppm. These metals are naturally concentrated in sulphide minerals, distributed in New Zealand as follows: mercury in eastern Northland; lead, arsenic, cadmium and mercury in the lead-zinc-copper and gold-silver deposits of the Hauraki region; and arsenic in gold deposits of west Nelson, Westland and Otago. These deposits are generally of small size and the heavy metal contents are commonly low, so that the influence of the heavy metals on the surface environment should be quite localised. We present the results of some geochemical prospecting surveys using arsenic, lead and mercury over areas of known sulphide mineralisation. The levels of arsenic and lead in soils and in stream sediments decrease sharply down-slope or down-drainage from known mineralisation. However, because the surveys were restricted to limited areas, with insufficient regional background data, full definition of the dispersion ranges of the heavy metals concerned is not possible.